Any beeps when you switch it on? Any changes to the PC lately, crashes (or more frequent crashes), blue screens? Any strange noises from the unit when it's on? Possibilites include overheating (and subsequent damage to the CPU) or failing power supply. If the guarantee has run out you can take the side off the machine and see if loads of dust and fluff has collected inside and around the fans, that would point to overheating, but it sounds as if the damage has already been done on that front.